Rational design and preparation of covalent organic frameworks and their functional mechanism analysis for lithium-ion and lithium sulfur/selenium cells

Lithium-ion (Li-ion) and lithium-sulfur/selenium (Li-S/Se) cell have attracted much attention acting as promising components for widely-used energy storage devices due to high theoretical specific capacity of metal lithium and good energy density. However, there are still some disadvantages in the process of commercialization including high cost and limited theoretical capacity of Li-ion cell, low conductivity and serious “shuttle effect” of lithium polysulfides/polyselenides of Li-S/Se cell. As typical porous and crystalline polymers, covalent organic frameworks (COFs) including nonmetal elements and accurately combined building blocks with special structures present outstanding applied potential for energy storage because of high porosity, controllable pore size and distribution, enough active sites for embedding Li+, open channels and π-conjugated systems for charge exchange and Li+ transportation and functional groups of COFs to strongly adsorb lithium polysulfides/polyselenides. Thus many novel COFs applied to novel cathodes, anodes, separators and interlayers are designed and prepared to resolve the problems mentioned above. In this review, the recent progresses on COFs used in Li-ion and Li-S/Se cells including preparation methods, design principles, working mechanisms of COFs in improving electrochemical performances and actual applications in the cells are summarized. Then we have a detailed discussion about applying COFs in the cells and construct structure-function relationships between COFs and performance. Finally, the existed challenges and future outlooks with COFs preparations, structure designs and practical applications in the cells are elaborated in detail. We hope the review can arouse more attention and lead more profound researches on COFs for energy storage.